1. Field of the Invention
Apparatuses and methods consistent with the present invention are directed to low noise amplification. More particularly, the present invention relates to a broadband low noise amplifier and a radiofrequency (RF) signal amplification method of the same.
2. Description of the Related Art
Generally, an amplifier is needed in order to amplify a signal that is output through an antenna when a wireless communication apparatus transmits a signal, or to provide proper gains to the amplitude of a signal input into an antenna. In the former case a power amplifier is required, and in the latter case a low noise amplifier is required.
The low noise amplifier amplifies, in a manner generating little noise, a weak radio frequency (RF) signal received by a wireless system through an antenna.
FIG. 1 illustrates an example of a wireless communication apparatus comprising a low noise amplifier. First, operations of the wireless communication apparatus relating to the transmission process will be described.
A baseband signal output from a baseband processor 190 is amplified by a baseband amplifier 140. The amplified baseband signal is mixed with an oscillation signal generated by an oscillator 180 in a up-mixer 130 to thereby create an RF signal. Most conventional communication systems do not directly convert a baseband signal into a RF signal. Instead, they first convert the baseband signal to an intermediate frequency (IF) signal and then convert the IF signal to an RF signal. The RF signal is amplified by a power amplifier 120 and then output through an antenna 110. The power amplifier 120 used in the transmission process may be constructed with multi-stage amplifiers so as to reduce distortion and obtain high gains. For example, a wireless communication apparatus may include a pre-power amplifier and a power amplifier.
Now, operations of the wireless communication apparatus relating to the reception process will be described.
The RF signal input through the antenna 110 is amplified by way of the low noise amplifier 150. The amplified RF signal is converted into a baseband signal by the down-mixer 160, and is amplified by the baseband amplifier 170. Most of the current conventional communication systems do not directly convert an RF signal into a baseband signal; they first convert the RF signal to an intermediate frequency (IF) signal and then convert the IF signal to a baseband signal. The amplified baseband signal is then transmitted to the baseband processor 190. The low noise amplifier 150 may also be constructed of multi-stage amplifiers so as to reduce distortion and obtain high gains.
A switch 115 selects between an RF signal output from the power amplifier 120 and intended for the antenna 110, and an RF signal received via the antenna 110 and destined for the low noise amplifier 150. In the full duplex mode communication system, a duplexer may be used instead of the switch 115.
As described above, the power amplifier or the low noise amplifier used in the wireless communication apparatus should provide sufficient variable gains.
The construction of the low noise amplifier 150 of the wireless communication apparatus illustrated in FIG. 1 is shown in FIG. 2.
The low noise amplifier shown in FIG. 2 has the structure of a differential cascade amplifier, comprising a first cascade amplifier including a common source transistor M1 and a common gate transistor M3, and a second cascade amplifier including a common source transistor M2 and a common gate transistor M4.
An RFin+ signal received via an antenna is supplied to the gate of the common source transistor M1 of the first cascade amplifier. The common gate transistor M3 improves the frequency response of the first cascade amplifier. The RFin+ signal passing through the first cascade amplifier is output through an output terminal (RFout+). As illustrated, the RFin+ input terminal is coupled to the common source transistor M1 using an inductor L1 for input matching and noise matching.
Likewise, an RFin− signal input through the same antenna is supplied to the gate of the common source transistor M2 of the second cascade amplifier. The common gate transistor M4 improves the frequency response of the second cascade amplifier. The RFin− signal passing through the second cascade amplifier is output through an output terminal (RFout−). As illustrated, the input terminal of RFin− is coupled to the common source transistor M2 using an inductor L2 for input matching and noise matching.
A gain control section A comprises a transistor M5 which functions as a switch and is provided with a switching signal Gtune5 to turn the transistor M5 on or off. Load 1 and load 2 are positioned at the output terminals, RFout+ and RFout−. Load 1 and load 2 are inductors, which act as loads of the low noise amplifier.
The RF signal received by the antenna is converted into a differential signal through a Balun (not shown) and then flows into an input terminal of the low noise amplifier. The input RF signal is amplified through the transistors M1 and M3 and the transistors M2 and M4, and is output through the output terminals RFout+and RFout−.
Input matching is conducted to reduce loss of an input signal, and output matching is conducted to reduce output loss. In addition, noise matching is conducted to reduce noise generation. Input matching and noise matching may be implemented by properly adjusting transistors M1 and M2 and inductors L1 and L2.
As described above, the conventional low noise amplifier has the properties of low noise and high gain, but it also has the property of narrow band in impedance matching.
In other words, since impedance matching in the form of an LC resonance is conducted at a load, the low noise amplifier has the highest gain at the resonance frequency, but its gain decreases at other frequencies.